Method for purifying a chlorine supply

ABSTRACT

This invention provides a method for purifying a chlorine supply that includes a chlorine component, a bromine component, and nitrogen trichloride. The method includes the steps of introducing the chlorine supply into a vaporizer, heating the chlorine supply in the vaporizer to form a vapor, and introducing the vapor into a distillation system to provide purified chlorine gas, a distillate that includes liquid chlorine and the bromine component, and a bottoms component including the nitrogen trichloride. The method also includes the steps of condensing the vapor in a reflux condenser, heating the condensate in a reboiler, removing the purified chlorine gas from the distillation system, and removing the distillate from the distillation system.

CROSS-REFERENCE TO RELATED APPLICATION

The subject patent application claims priority to, and all the benefitsof, U.S. Provisional Patent Application Ser. No. 61/261,176 filed onNov. 13, 2009. The entirety of this provisional patent application isexpressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to a method for purifying achlorine supply including a chlorine component, a bromine component, andnitrogen trichloride. More specifically, the method includes utilizing aparticular distillation system to form purified chlorine gas and todecompose the nitrogen trichloride.

DESCRIPTION OF THE RELATED ART

Chlorine gas is typically commercially produced using one or more wellknown electrolysis processes such as mercury cell electrolysis,diaphragm cell electrolysis, membrane cell electrolysis, and/orelectrolysis of fused chloride salts according to the Downs Process. Theelectrolysis processes typically produce chlorine throughelectrochemical reactions in brine solutions (e.g. NaCl and KClsolutions) as follows:Cathode: 2H⁺(aq)+2e ⁻→H₂(g)Anode: 2Cl⁻(aq)→Cl₂(g)+2e ⁻Overall process: 2NaCl(or KCl)+2H₂O→Cl₂(g)+H₂+2NaOH(or KOH)After formation, the chlorine gas can be treated with water and/or steamand then dried by cooling or treatment with sulfuric acid to minimizechlorine hydrate formation.

At various points during the formation of chlorine gas, nitrogentrichloride (NCl₃) is also typically formed. It is believed that NCl₃forms from side reactions of chlorine atoms and anhydrous ammonia orammonium salts (e.g. ammonium hydroxide, ammonium chloride, and ammoniumsulfate) that are present at one or more points in the process. Theseside reactions typically occur as follows:NH₃+3Cl₂→NCl₃+3H⁺+3Cl⁻NH₄ ⁺+3Cl₂→NCl₃+4H⁺+3Cl⁻NH₄ ⁺+Cl⁻+3HClO→NCl₃+H⁺+Cl⁻+3H₂OThe formation of nitrogen trichloride typically occurs due to brinecontamination, steam contamination, and/or water contamination. Any ureathat is present in the brine, steam, or water can hydrolyze to formammonium which can then be converted into nitrogen trichloride.Alternatively, salts used to form the brine can be contaminated withammonium nitrate that can be converted into nitrogen trichloride. Sodiumhydroxide that is typically used to form the brine can also becontaminated with ammonia depending on purification processes. In somecases, sulfuric acid used to dry the chlorine gas can be contaminatedwith ammonia. In still other cases, direct contact cooling water orsteam can be treated with amines, ammonia based flocculants, orchloramines which can lead to formation of nitrogen trichloride. Evenground water can include ammonia compounds that can be converted intonitrogen trichloride.

As is well known in the art, nitrogen trichloride is sensitive to heat,light, sound, and shock and can quickly degrade at a rate sufficient tocause an explosion. Accordingly, nitrogen trichloride is preferablyremoved from chlorine gas but is typically done so in a complex, timeconsuming, and expensive manner. As set forth in FIG. 1, whichrepresents the prior art, dried chlorine gas formed from electrolysis istypically washed with liquid chlorine in a washing column to minimize anamount of the nitrogen trichloride and cool the chlorine gas therebyincreasing safety. In addition, the washing column also separateschlorinated organic compounds from the chlorine gas thereby increasingthe purity of the chlorine gas. The washing column is typicallyconnected to an external condenser and reboiler to increase theefficiency of the washing process, as also set forth in FIG. 1. In thewashing column, the nitrogen trichloride and the various chlorinatedorganic compounds typically condense or dissolve in the liquid chlorineand may be recycled through the external condenser and reboiler, asdescribed above. The reboiler is typically operated cold (0° C.-5° C.)or hot (45° C.-60° C.) and can act as a storage vessel for the nitrogentrichloride or as a point of decomposition. Carbon tetrachloride (CCl₄)is typically added to the washing column to extract the nitrogentrichloride and allow for its removal from the washing column andsubsequent disposal. Upon addition of the carbon tetrachloride, thenitrogen trichloride is separated from the chlorine, which is vaporizedin the reboiler and returned to the washing column. After the chlorinegas is washed and separated from the nitrogen trichloride and thevarious chlorinated organic compounds, the chlorine gas is typicallycompressed using liquid ring compression, reciprocating compression, orcentrifugal compression, cooled using inter- and after-coolers, and thenliquefied into liquid chlorine. The liquid chlorine then can be scrubbedand sold commercially. However, even after drying, washing, compression,cooling, and liquefaction, trace amounts of both the carbontetrachloride and the nitrogen trichloride, in addition to trace amountsof scrubbing compounds, typically leach into the liquid chlorine and actas impurities when the liquid chlorine and/or chlorine gas is used indownstream commercial synthetic processes.

In addition to the nitrogen trichloride, commercial production ofchlorine gas tends to produce a variety of byproducts includingmolecular bromine (Br₂), bromine-chloride (Br—Cl), and various organiccompounds. These byproducts, in addition to the carbon tetrachloride andthe nitrogen trichloride, are also impurities when the chlorine gas isused in commercial processes. As is known in the art, when chlorine gasis used to synthesize phosgene, which in turn is used to synthesizeisocyanates, presence of the carbon tetrachloride, nitrogen trichloride,and brominated compounds typically add color to the isocyanates whichmakes the isocyanates less commercially desirable. Accordingly, thesebyproducts are typically removed using distillation and other separationtechniques because chlorine gas is more volatile than many of thebyproducts. However, entire chlorine streams are typically evaporated toachieve such distillations. One example of a distillation process isschematically set forth in FIG. 1, as first introduced above. In thisdistillation process, and in many similar processes, external condensersand reboilers are used and connected via long lengths of pipes to formeffective distillation systems. However, use of these types of systemsis very expensive, untimely, and complex. In addition, the long lengthsof pipes used in these systems only increase a number of points at whichthe systems can fail, thus increasing safety risks and concerns.Furthermore, many of these systems also fail to effectively reduceamounts of nitrogen trichloride to sufficient levels.

Some distillation systems utilize high pressure steam heated reboilerswhich have a tendency to lead to film boiling, hot spots, andsuperheated chlorine gas, all of which are undesirable. Moreover, manydistillation systems have a tendency to suffer from chlorine andnitrogen trichloride “holdup,” i.e., accumulation of excess amounts ofchlorine gas and nitrogen trichloride in the distillation systems, whichleads to safety and environmental concerns.

Other distillation systems do not effectively control amounts ofincoming chlorine in relation to the efficiency of distillation andseparation of desired compounds. This lack of control tends to reduceefficiency of the distillation systems and does not allow forcustomization of distillation processes to maximize separation ofdesired compounds. In addition, this lack of control contributes to theholdup of the chlorine and nitrogen trichloride, thus further increasingsafety concerns.

Accordingly, there remains an opportunity to develop a cost-effectiveand energy efficient method for removing by-products from a chlorinesupply that can be used safely and with decreased environmentalconcerns.

SUMMARY OF THE INVENTION AND ADVANTAGES

The instant invention provides a method for purifying a chlorine supply.The chlorine supply includes a chlorine component, a bromine component,and nitrogen trichloride. The chlorine supply is purified in adistillation system to form purified chlorine gas having less than 20parts by weight of the bromine component per one million parts by weightof the purified chlorine gas and to form a distillate comprising liquidchlorine and the bromine component. The distillation system is fluidlyconnected to a vaporizer and includes a distillation tower that has anupper end, a lower end, a vertical axis extending through the upper andlower ends, and a vapor-liquid contact device to provide a vapor-liquidinterface between a vapor and a condensate. The distillation system alsoincludes a reflux condenser disposed above the distillation tower and influid communication with the upper end of the distillation tower. Thereflux condenser shares the vertical axis with the distillation tower.The distillation system also includes a reboiler disposed below thedistillation tower and in fluid communication with the lower end of thedistillation tower. The method includes the steps of introducing thechlorine supply into the vaporizer, heating the chlorine supply in thevaporizer to form the vapor, and introducing the vapor into thedistillation system to provide purified chlorine gas, a distillateincluding the liquid chlorine and the bromine component, and a bottomscomponent comprising the nitrogen trichloride. The method also includesthe step of condensing the vapor in the reflux condenser to form thecondensate which flows from the reflux condenser into the upper end ofthe distillation tower such that the condensate interacts with the vaporat vapor-liquid contact device thereby forming the purified chlorine gasand the distillate. Furthermore, the method includes the steps ofheating the condensate in the reboiler, removing the purified chlorinegas from the distillation system, and removing the distillate from thedistillation system.

The method of this invention purifies the chlorine supply with increasedenergy efficiency, increased cost savings, and increased safety, therebyreducing environmental concerns. This method also efficiently separateschlorine gas from various by-products while simultaneously reducingholdup of the nitrogen trichloride, liquid chlorine and the brominecomponent. This method also increases safety and reduces environmentalconcerns associated with distillation tower design through reduced useof piping and external condensers and reboilers.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated,as the same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a schematic of a chlorine purification system of the prior artwherein chlorine gas is formed from electrolysis, filtered, dried,washed and then purified in a distillation system including adistillation tower and a condenser and reboiler that are disposed apartfrom the distillation tower;

FIG. 2 is a schematic of a typical distillation tower of the prior artincluding a condenser, pump, and reboiler that are disposed apart fromthe distillation tower and long lengths of piping disposed therebetween;

FIG. 3 is a side view of one embodiment of the distillation tower of theinstant invention having an upper and a lower end wherein a width of theupper end is greater than a width of the lower end;

FIG. 4 is a schematic of one embodiment of the instant invention showinga distillation system including a distillation tower, a reflux condenserdisposed above the distillation tower, in fluid communication with anupper end of the distillation tower, and sharing a vertical axis withthe distillation tower to condense a vapor into a condensate such thatthe condensate flows into the upper end of the distillation tower, and areboiler disposed below the distillation tower and in fluidcommunication with the lower end of the distillation tower to heat thecondensate;

FIG. 5 is a schematic of another embodiment of the instant inventionshowing the distillation system of FIG. 4 in fluid communication with aneutralization tower; and

FIG. 6 is a side view of an embodiment of the neutralization towerhaving a first end and a second end wherein a width of the first end isgreater than a width of the second end.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for purifying a chlorine (Cl₂)supply and producing a purified chlorine gas (also known as an “overheadstream” in the art) in a distillation system (10). The purified chlorinegas is formed via distillation/fractionation procedures. The chlorinesupply is not particularly limited and is not dependent on anyparticular method of formation. The chlorine supply may include liquidchlorine (Cl_(2(l))) or gaseous chlorine (Cl_(2(g))) such as those typesused in commercial or industrial applications. The chlorine supply (alsoknown as a “feed in” in the art) may be a finite supply or a continuoussupply and may be provided from continuous or batch chlorine supplyprocesses and/or in discrete units, such as from commercial tankertrunks or locomotive tanks. In one embodiment, the chlorine supply isprovided in a semi-batch process. Typically, the chlorine supply isprovided as a liquid via a pipeline to one or more storage locations andthen purified and used. However, even when provided as a liquid, somechlorine gas tends to be present. The pipeline is typically suppliedfrom railcars. Alternatively, the chlorine supply may be provided fromas a gas or a mixture of a liquid and a gas.

The chlorine supply includes a chlorine component which may include,consist essentially of, or consist of liquid chlorine (Cl_(2(l)))gaseous chlorine (Cl_(2(g))), ferric chloride (iron (III) chloride),and/or chlorinated hydrocarbons such as carbon tetrachloride,chloroform, and methylene chloride. The terminology “consistingessentially of” limits the chlorine component, in one embodiment, fromincluding an amount any other compound, such as organic compounds, thatmaterially affects the basic and novel characteristics of the chlorinecomponent. It is to be understood that trace amounts of these othercompounds, such as trace amounts of the organic compounds, can beincluded so long as the trace amounts do not materially affects thebasic and novel characteristics of the chlorine component. The chlorinesupply also includes nitrogen trichloride (NCl₃). The nitrogentrichloride may be present as a liquid or a gas or as a mixture of aliquid and a gas.

Further, the chlorine supply also includes a bromine component which mayinclude, consist essentially of, or consist of liquid bromine(Br_(2(l))), gaseous bromine (Br_(2(g))), bromine monochloride (Br—CL),and/or brominated hydrocarbons. The terminology “consisting essentiallyof” limits the bromine component, in one embodiment, from including anyother compound, such as organic compounds, that materially affects thebasic and novel characteristics of the bromine component. It is to beunderstood that trace amounts of these other compounds, such as traceamounts of the organic compounds, can be included so long as the traceamounts do not materially affects the basic and novel characteristics ofthe bromine component. In one embodiment, the bromine component includesliquid bromine (Br_(2(l))), gaseous bromine (Br_(2(g))), and brominemonochloride (Br—Cl).

The chlorine supply typically includes greater than 95, more typicallygreater than 97, still more typically greater than 99, even moretypically greater than 99.5, and most typically at least 99.9, parts byweight of the chlorine component per 100 parts by weight of the chlorinesupply. The chlorine supply also typically includes from 1 to 500, moretypically of from 1 to 300, still more typically from 50-300, and evenmore typically from 50 to 200, parts by weight of the bromine componentper one million parts by weight of the chlorine supply. In oneembodiment, the chlorine supply includes approximately 200 parts byweight of the bromine component per one million parts by weight of thechlorine supply. The chlorine supply further typically includes lessthan about 5 and more typically of from 2 to 5, parts by weight of thenitrogen trichloride per one million parts by weight of the chlorinesupply. The chlorine supply may also include water. In variousembodiments, the water is present in an amount of less than about 30,and more typically in amounts of from 5 to 25, parts by weight per onemillion parts by weight of the chlorine supply.

In one embodiment, the chlorine supply includes the chlorine component,the bromine component, and the nitrogen trichloride. In anotherembodiment, the chlorine supply includes the chlorine component, thebromine component, the nitrogen trichloride, and the water. In stillanother embodiment, the chlorine supply consists essentially of thechlorine component, the bromine component, and the nitrogen trichloride.In yet another embodiment, the chlorine supply consists essentially ofthe chlorine component, the bromine component, the nitrogen trichloride,and the water. In these embodiments, the terminology “consistingessentially of” limits the chlorine supply from including any othercompound, such as organic compounds, that materially affects the basicand novel characteristics of the chlorine component, the brominecomponent, the nitrogen trichloride, and/or the water. It is to beunderstood that trace amounts of these other compounds, such as traceamounts of the organic compounds, can be included so long as the traceamounts do not materially affects the basic and novel characteristics ofthe chlorine component, the bromine component, the nitrogen trichloride,and/or the water. In a further embodiment, the chlorine supply consistsof the chlorine component, the bromine component, and the nitrogentrichloride. In still another embodiment, the chlorine supply consistsof the chlorine component, the bromine component, the nitrogentrichloride, and the water.

In addition to the chlorine component, the bromine component, thenitrogen trichloride, and/or the water, the chlorine supply may alsoinclude carbon tetrachloride (CCl₄), as described above. In variousembodiments, the chlorine supply typically includes less than about 20,more typically less than about 15, and most typically of about 10, partsby weight of the carbon tetrachloride per one million parts by weight ofthe chlorine supply.

Although the chlorine supply may include chlorine component, the brominecomponent, the nitrogen trichloride, the water, and/or carbontetrachloride (CCl₄), the purified chlorine gas formed in the method ofthis invention typically includes less than 20, more typically less than15, still more typically less than 10, and even more typically less than5, parts by weight of the bromine component per one millions parts byweight of the purified chlorine gas. It is also contemplated that thepurified chlorine gas may include about 4, 3, 2, or 1, part by weight ofthe bromine component per one millions parts by weight of the purifiedchlorine gas.

The method for purifying the chlorine supply includes the steps ofintroducing the chlorine supply into a vaporizer (12) and heating thechlorine supply in the vaporizer (12) to form a vapor. Typically, thechlorine supply is pumped from a storage unit at a temperature of from0° C. to 20° C., from 5° C. to 20° C., from 5° C. to 15° C., or at about10° C., into the vaporizer (12). The chlorine supply is also typicallypumped at a pressure of from 1 to 20, from 5 to 15, from 10 to 15, orfrom about 12 to about 14, bar, into the vaporizer (12). The chlorinesupply is typically introduced into the vaporizer (12) as a liquid butmay be introduced into the vaporizer (12) as a mixture of a liquid and agas.

The vaporizer (12) may be any known in the art and typically operatesvia thermal conduction, convection, or thermal radiation. Typically, thevaporizer (12) heats the chlorine supply beyond its boiling point toform the (chlorine) vapor. Most typically, the vaporizer (12) heats thechlorine supply to a temperature of from 50° C. to 120° C., even moretypically of from 70° C. to 110° C., and still more typically of from90° C. to 100° C., and most typically of from 95° C. to 100° C. In oneembodiment, the vaporizer (12) heats the chlorine supply to atemperature of about 92° C. to 98° C. The step of heating the chlorinesupply may be further defined as vaporizing and super-heating thechlorine supply to facilitate decomposition of nitrogen trichloride.Without intending to be bound by any particular theory, it is believedthat at temperatures above 121° C., carbon steel experiences a rapidincrease in corrosion, as is well known in the European ChlorineIndustry (“Euro Chlor”) and the Chlorine Institute. Typically, thevaporizer (12) forms the vapor at a pressure of 5 to 20, from 10 to 20,or from 10 to 15, bar. In one embodiment, the vaporizer (12) forms thevapor at a pressure of from about 10 to 14 bar.

In another embodiment, flow of the chlorine supply into the vaporizer(12) is controlled by pressure. Liquid chlorine may be vaporized at 12.5bar gage and superheated to 95° C. through use of steam at 1 to 12, 1 to5, or 1 to 2 bar. Most preferably, the steam is pressurized to 1.2 barsuch that that liquid chlorine, and chlorine vapor, do not reach atemperature above 120° C. It is believe that at temperatures above 120°C., chlorine-iron reactivity increases substantially. Typically, thesteam is automatically throttled to achieve a chlorine superheattemperature of from about 85° C. to 105° C. In one embodiment, anoperating pressure of from about 10 to 14 bar is required to ensureadequate performance of a distillation tower (14).

The method also includes introducing the vapor into the distillationsystem (10) that is fluidly connected to the vaporizer (12). The vaporis typically introduced into the distillation system (10) at a pressureof from 5 to 20, from 10 to 20, or from 10 to 15, bar. In oneembodiment, the vapor is introduced into the distillation system (10) ata pressure of from about 10 to 14 bar. The vapor is typically introducedinto the distillation tower (14) through a vapor input (not shown in theFigures). The vapor input may be any type of valve, regulator, nozzle,spout, faucet, or the like. The vapor input typically fluidly connectsthe distillation tower (14) and the vaporizer (12) through a vapor line,pipe, or tube. The vapor input can be controlled with a flow controllingdevice that is described in greater detail below.

Typically, the step of introducing the vapor into the distillationsystem (10) produces liquid chlorine and purified chlorine gas in thedistillation system (10). The step of introducing the vapor into thedistillation system (10) also typically produces an overhead vaporincluding up to about 5 ppmw of the bromine component and a bottomsproduct of the chlorine component including up to about 15 wt % of thebromine component. The distillation system (10) may be any known in theart suitable for distillation and/or fractionation purposes. Typically,the distillation system (10) includes the distillation tower (14), firstdescribed above, which is also known in the art as a fractionationtower. The distillation system (10) also includes a reflux condenser(24) and a reboiler (26), which are described in greater detail below.The distillation system (10) may also include the flow controllingdevice. The distillation tower (14) may have one or more than one valve,control device, regulator, nozzle, spout, faucet, or the like, differentfrom the vapor input, for adding or removing any component from thedistillation tower (14) as desired by one of skill in the art.

The distillation tower (14) may be of any size and shape and may bedesigned using the McCabe-Thiele method, the Fenske equation, or varioussimulation models, as are well known in the art. Typically, thedistillation tower (14) has a shape as set forth in FIG. 3, but is notlimited to this shape. The distillation tower (14) is typically a columnhaving an upper end (16) and a lower end (18). The upper end (16) andlower end (18) typically have varying widths, e.g. diameters (D₁, D₂,respectively). In other words, the distillation tower (14) typically hasvarying widths at different points along the tower, as shown in FIGS.3-5.

In various embodiments, a ratio of a width of the upper end (16) to awidth of the lower end (18) is about 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1,or 8:1. It is also contemplated that these ratios may vary ±60%, ±50%,±40%, ±30%, ±25%, ±20%, ±10%, ±5%. It is also contemplated that anyrange therebetween one or more of the aforementioned values may also beutilized. The distillation tower (14) typically has a height of from 4to 30 meters (13 to 98 feet). In various embodiments, the distillationtower (14) has a height of from 9 to 21 meters (30 to 70 feet), moretypically of from 9 to 18 meters (30 to 60 feet), and most typically offrom 12 to 15 meters (40 to 50 feet). In one embodiment, the upper end(16) has a diameter of greater than or equal to 0.5 meters and the lowerend (18) has a diameter less than 0.5 meters. In various embodiments,the distillation tower (14) has a width and/or height ±60%, ±50%, ±40%,±30%, ±25%, ±20%, ±10%, ±5%, or any range therebetween, of theaforementioned values. Typically, the diameter and height of thedistillation tower (14) vary depending on a variety of factorsincluding, but not limited to, volatility differences in components,amounts/rates of bromine and chlorine to be separated, a purity of theincoming chlorine supply, operating temperatures, energy requirements,desired separation levels, and the like. Also, the height of thedistillation tower (14) typically varies depending on a number oftheoretical plates needed and a quality of distillation needed, asselected by one of skill in the art. Accordingly, one or more of thesefactors may be manipulated in the instant invention as determined bythose of skill in the art. Typically, one or more of these factors ismanipulated based on simulations derived from the McCabe-Thiele methodand/or the Fenske equation which can be used to customize dimensions ofthe distillation tower (14).

The distillation tower (14) has an upper end (16) and a lower end (18)and both are typically cylindrical, as shown in FIG. 3. The upper andlower ends (16, 18) may be right circular cylinders, ellipticalcylinders, parabolic cylinders, or hyperbolic cylinders. The upper andlower ends (16, 18) may have the same shape or different shapes andneither must be cylindrical. Said differently, the upper and lower ends(16, 18) can be of any shape. Typically the upper end (16) is a rightcircular cylinder that has a greater diameter than the lower end (18)that is also typically a right circular cylinder, as shown in FIG. 3.

The upper end (16) is disposed above the lower end (18) relative togravity and the earth. The distillation tower (14) typically has avertical axis (V₁) that extends through the upper and lower ends (16,18), as shown in FIG. 3. Typically, the upper and lower ends (16, 18)extend along the vertical axis (V₁). In one embodiment, the distillationtower (14) has two pairs of shoulders that extend radially from thevertical axis. A first pair of shoulders (34) is typically near theupper end (16) and forms an acute angle with the vertical axis. A secondpair of shoulders (36) is typically disposed at the lower end (18) andforms an obtuse angle with the vertical axis (V₁). The distillationtower (14) also typically has a horizontal axis (H₁) that extendsbetween the upper and lower ends (18), as also shown in FIG. 3.Additionally, the upper end (16) of the distillation tower (14)typically defines a first orifice (not shown) for removing the purifiedchlorine gas while the lower end (18) of the distillation tower (14)typically defines a second orifice (not shown) for removing othercomponents produced from this method, as described in greater detailbelow.

The distillation tower (14) also typically includes at least onevapor-liquid contact device. In one embodiment, the at least onevapor-liquid contact device is disposed substantially transverse to thevertical axis (V₁) and substantially parallel to the horizontal axis(H₁). The at least one vapor-liquid contact device provides avapor-liquid interface between the vapor rising up the distillationtower (14) and a condensate flowing down the distillation tower (14)from the reflux condenser (24). It is contemplated that the condensatemay include purified chlorine and portions of a distillate, described ingreater detail below. The condensate and the reflux condenser (24) aredescribed in greater detail below. At the vapor-liquid interface, avapor-liquid equilibrium exists such that the condensate purifies thevapor by absorbing impurities. Without intending to be bound by anyparticular theory, it is believed that the vapor-liquid interface in thedistillation tower (14) allows liquid chlorine to “scrub” chlorinevapor. More specifically, the liquid chlorine interacts with the vaporrising from the reboiler (26). Interaction of the vapor and thecondensate is believed to form the purified chlorine gas through bothrectification and separation. In other words, the condensate providesnecessary cooling to condense the vapor rising from the reboiler (26),thereby increasing the effectiveness of the distillation tower (14).Increased interaction of the condensate and the vapor increases theeffectiveness of the distillation tower (14) and the formation of thepurified chlorine gas. In addition to forming the purified chlorine gas,the condensate typically interacts with the vapor at the vapor-liquidcontact device to form the distillate (also known as a “feed out” in theart). Both the purified chlorine gas and the distillate can be removedfrom the distillation tower (14).

The vapor-liquid contact device may be any known in the art andtypically includes a plurality of trays (20) and/or packing material(22), as is well known in the art. In one embodiment, the distillationtower (14) includes a first vapor-liquid contact device within the upperend (16) and a second vapor-liquid contact device within the lower end(18). In this embodiment, the first vapor-liquid contact device istypically further defined as a body of packing material (22) while thesecond vapor-liquid contact device is typically further defined as aplurality of trays (20), as shown in FIG. 3.

Typically, the vapor condenses on the plurality of trays (20) and/orpacking material (22) and runs down the distillation tower (14). Thetray (20) that has the highest temperature is typically disposed in thedistillation tower (14) in a lowermost position relative to the ground.The tray (20) with the lowest temperature is typically disposed in anuppermost position relative to the ground. As is well known in the art,at steady state conditions, vapor and liquid on each tray (20) are atthermal equilibrium.

The plurality of trays (20) may include any number of trays (20), asdetermined by one of skill in the art. However, the plurality of trays(20) typically includes from 3 to 30, from 5 to 25, from 5 to 20, from 5to 15, or from 5 to 10, trays (20). It is also contemplated that anynumber of trays (20) within the aforementioned ranges may be utilized.Moreover, any type of tray (20) may be used in the instant invention, asselected by one of skill in the art. In one embodiment, the plurality oftrays (20) is disposed substantially transverse to the vertical axis(V₁) and substantially parallel to the horizontal axis (H₁). In anotherembodiment, the plurality of trays (20) is further defined as horizontaltrays (20).

Typically, each of the plurality of trays (20) has a diameter of from0.25 to 0.76 meters (10 to 30 inches), from 0.25 to 0.50 meters (10 to20 inches), or from 0.38 to 0.64 meters (15 to 25 inches). However, eachof the plurality of trays (20) is not limited in size and shape.Moreover, each of the plurality of trays (20) is typically spaced fromone another at a distance of from 0.25 to 0.76 meters (10 to 30 inches),from 0.25 to 0.50 meters (10 to 20 inches), or from 0.38 to 0.64 meters(15 to 25 inches). Again, this distance is not limited and may be variedby one of skill in the art. It is also contemplated that the diameterand spacing described above may vary ±60%, ±50%, ±40%, ±30%, ±25%, ±20%,±10%, ±5%, or any range therebetween, of the aforementioned values.

The packing material (22) is typically utilized when low pressure dropsacross the distillation tower (14) are used, such as when a vacuum isemployed. In one embodiment, the packing material (22) is used to theexclusion of the trays (20). In another embodiment, both packingmaterial (22) and trays (20) are used. Typically, the packing material(22) is random dumped packing material (22) (e.g. 0.3-0.9 meters (1-3feet) wide) such as Raschig rings or structured sheet metal. Liquidsformed in the distillation tower (14), such as the condensate and thedistillate, typically wet the packing material (22) while the vaporpasses across the packing material (22), thus allowing mass transfer andpurification of the vapor to take place. As is well known in the art, avapor liquid equilibrium curve forms when using packing material (22) iscontinuous, as opposed to when a plurality of trays (20) is utilizedwherein each tray (20) represents a separate point of vapor liquidequilibrium.

To maximize surface area per unit volume for an optimal liquid vaporinterface, structured packing material (22) can be utilized in the upperportion of the distillation tower (14). As is known in the art,selection of ideal packing material (22) size involves a balancingbetween maximum efficiency and maximum capacity. In one embodiment ofthis invention, Flexipac 1YHC structure packing material (22) isutilized. Alternately a variety of random and/or dump packing material(22) may be utilized as well. The packing material (22) is notparticularly limited in this invention and may include any known in theart. In one embodiment, the packing material (22) includes stainlesssteel, ceramic, nickel, chromium, manganese, iron, and/or chlorinestable polymers such as perfluorinated polymers (e.g. PTFE). In anotherembodiment, the packing material (22) is commercially available underthe trade name Hastelloy.

Typically, the packing material (22) has a surface area to volume ratioof at least 50:1 and preferably higher. In various embodiments, thepacking material (22) has a surface area to volume ratio of about 75:1,100:1, 150:1, 200:1, 250:1, 300:1, 350:1, 400:1, 450:1, or 500:1. It isalso contemplated that any ratio of surface area to volume within theaforementioned ratios may be utilized. The packing material (22) can bedisposed within the distillation tower (14) in any amount. Typically,the amount of the packing material (22) disposed in the distillationtower (14) is optimized based on simulations derived from theMcCabe-Thiele method and/or the Fenske equation. In various embodiments,the packing material (22) is disposed in the distillation tower (14) ata height of from 1.5 to 7.6 meters (5 to 25 feet), from 3 to 7.6 meters(10 to 25 feet), from 3 to 6 meters (10 to 20 feet), or from 3.6 to 4.9meters (12 to 16 feet). It is also contemplated that the heightdescribed immediately above may vary ±60%, ±50%, ±40%, ±30%, ±25%, ±20%,±10%, ±5%, or any range therebetween, of the aforementioned values.

The packing material (22) of this invention typically reduces the dragof the vapor on the condensate in the distillation tower allowing thecondensate to move down the column to successively lower trays (20)without excessive accumulation. The packing material (22) typicallyreduces any liquid hold-up at the vapor-liquid interface and an openarea for vapor flow is increased. The condensate and vapor typicallyflow in vertical countercurrent directions within the distillation tower(14) reducing a shear effect at a bottom edge of the packing material(22). This is thought to result in a significantly higher floodingcapacity and a reduced pressure drop in a loading region of the packingmaterial (22). In addition, a vertical region at the vapor-liquidinterface tends to enlarge a turning radius for vapor flow and tends toreduce pressure drop associated with rotation between the plurality oftrays (20). A smooth transition between the plurality of trays (20)enhances vapor handling capacity of the distillation tower (14). In oneembodiment, increased column efficiency can be realized without a lossin capacity through use of packing material (22) with decreased crimpsizes with lower HETP and higher NTSM.

Referring now to the reflux condenser (24), this condenser is disposedabove the distillation tower (14) and is typically flanged to thedistillation tower (14). The reflux condenser (24) typically shares thevertical axis (V₁) of the distillation tower (14). The reflux condenser(24) may be disposed in direct contact with the distillation tower (14)or may be disposed apart from the distillation tower (14). In oneembodiment, the reflux condenser (24) is disposed on, and in directcontact with, the first pair of shoulders (34) of the central body. Thereflux condenser (24) typically has a height of from 0.3 to 3 meters (1to 10 feet), of from 0.9 to 3 meters (3 to 10 feet), or from 1.8 to 3meters (6 to 10 feet). The reflux condenser (24) also typically has adiameter equal to or slightly larger than the diameter of the upper end(16) of the distillation tower (14). In various embodiments, the refluxcondenser (24) has a diameter of from 0.2 to 2.5 meters (10 to 100inches), of from 0.5 to 2 meters (20 to 80 inches), or of from 0.6 to 1meter (25 to 40 inches). In one embodiment, the reflux condenser (24)has a diameter of about 0.6 to 0.9 meters (25 to 35 inches). It is alsocontemplated that the height and diameters described above may vary±60%, ±50%, ±40%, ±30%, ±25%, ±20%, ±10%, ±5%, or any rangetherebetween, of the aforementioned values.

The reflux condenser (24), first introduced above, is typically in fluidcommunication with the upper end (16) of the distillation tower (14) tocondense the vapor and form the condensate, introduced above. Typically,the condensate flows (e.g. is distributed) into the upper end (16) ofthe distillation tower (14). The reflux condenser (24) is typicallydisposed in direct contact with the distillation tower (14) such thatthe condensate flows directly back into the upper end (16) of thedistillation tower (14). This allows for minimized tubing and piping tobe used, thereby increasing safety, efficiency, and cost-effectiveness.

The reflux condenser (24) is typically further defined as close-coupledand may be further defined as an integral overhead reflux condenser (24)or an overhead internal knockback condenser. The knockback condenser maybe any known in the art and may be an upflow or a downflow knockbackcondenser. As is known in the art, knockback condensers typicallyutilize vapor risers to introduce a flow of the vapor into a headspaceabove a heat exchanger thereby establishing a flow of condensate. In theinstant invention, the flow of condensate is typically distributeddirectly back into the upper end (16) of the distillation tower (14), asdescribed above, while a portion is removed as the distillate, describedin greater detail below. In one embodiment, the reflux condenser (24) iscooled using liquid coolant, which may be any known in the art. As isknown in the art, portions of the vapor that are not refluxed as thecondensate can be removed as the purified chlorine gas. The purifiedchlorine gas may be removed from the distillation system (10) at anytemperature equal to or higher than the boiling point of chlorine undertower operating pressures.

Referring back to the reboiler (26) described above, the reboiler (26)is typically disposed below the distillation tower (14) and in fluidcommunication with the lower end (18) of the distillation tower (14).The reboiler (26) can be used to heat the condensate and decompose thenitrogen trichloride, but such step is not required in the instantinvention. In one embodiment, the temperature of the reboiler (26) isbelow a temperature of decomposition of the nitrogen trichloride. As iswell known in the art, reboilers (26) are heat exchangers and aretypically used to provide heat to the bottom of the distillation tower(14). In one embodiment, the reboiler (26) provides heat to thedistillation tower (14) such that the upper end (16) of the distillationtower (14) is maintained at a temperature of about 90° C. to 110° C.while the lower end (18) is maintained at a temperature of about 70° C.to 90° C. In another embodiment, the upper end (16) and the lower end(18) are maintained at temperatures that differ by about 1° C., 3° C.,5° C., 10° C., 15° C., 20° C., or 25° C. In other words, the reboiler(26) uses energy to drive the vapor up the distillation tower (14).Thus, the total amount of vapor in the distillation tower (14) is afunction of the vapor entering from the vaporizer through the vaporinput and the vapor formed from the reboiler (26). In one embodiment,the bottom of the distillation tower operates at a slightly highertemperature than the top due to an elevated content of high boilingpoint components and the use of superheated steam operating atsaturation temperatures. In various embodiments, the reboiler (26) ofthis invention is coupled (e.g. electronically) with the refluxcondenser (24) and the vapor input to control the amount of vaporentering the distillation tower (14). Typically, the reboiler (26) is atype of heat exchanger (e.g. schell and tube or plate and frame) thatincludes a heated fluid. The heated fluid is passed over or next to aliquid (such as the condensate) in the distillation tower (14) andexchanges energy or heat with the liquid without liquid/fluid contact.

The reboiler (26) of this invention typically vaporizes or boils thecondensate and may simultaneously decompose the nitrogen trichloridewhile forming and heating the vapor in the distillation tower (14).However, the vapor can be heated in the distillation tower (14) by anymeans known in the art. The vapor may be heated via other meansincluding through heating means disposed in contact with thedistillation tower (14), other than the reboiler (26). In oneembodiment, the vapor in the distillation system (10) is heated byadditional chlorine vapor entering the distillation system (10).

In one embodiment, the heat of the reboiler (26) decomposes the nitrogentrichloride safely and in a manner that reduces a total amount ofnitrogen trichloride present in the distillation system (10) at any onetime. Without intending to be bound by any particular theory, it isbelieved that the nitrogen trichloride decomposes pursuant to thefollowing equations such that inert nitrogen is formed and chlorine gasis formed that re-enters the distillation tower (14) and can beincorporated into the purified chlorine gas:NCl₃→.NCl₂+Cl.(Initiation)Cl₂→2Cl.(Initiation)Cl.+NCl₃→.NCl₂+Cl₂(Propagation).NCl₂+NCl₃→N₂+2Cl₂+.Cl(Propagation)

The reboiler (26) may be any type known in the art and may operate viathermal conduction, convection, or thermal radiation. In one embodiment,the reboiler (26) is steam operated. In another embodiment, the reboiler(26) utilizes hot oil or a synthetic organic heat transfer fluid such asDowtherm®, commercially available from the Dow Chemical Company. Instill another embodiment, low pressure steam (e.g. steam at a pressureof from 1.2 to 10 bar) is used which minimizes the possibility that thecondensate will reach a temperature in excess of 120° C. Typically, lowpressure steam is recovered from a higher pressure steam condensate andthen redistributed. Alternately, steam at a pressure of greater than 1.2bar can be used in a cross exchanger. Typically, the reboiler (26) isfurther defined as a shell and tube heat exchanger. However, the instantinvention is not limited to such a reboiler (26). It is contemplatedthat the reboiler (26) may be further defined as a kettle reboiler (26),a thermosyphon reboiler (26), a fired reboiler (26), or a forcedcirculation reboiler (26). In one embodiment, the reboiler (26) isheated with steam at 1 to 3 bar that is controlled or throttled toachieve a desired balance of components in the distillation tower (14).

In various embodiments, a specified amount of various boiling liquids ismaintained in the reboiler (26). These liquids can require furtherpurification or can be treated as waste. Typically, the level of liquidsis maintained in reboiler (26) via a level control valve thatmanipulates the flow out of reboiler (26) to either an additionaldistillation system or an impurity destruction device.

In one alternative embodiment, the reflux condenser (24), the reboiler(26), and the vapor input are coupled to a flow controlling device (notshown in Figures) for controlling a rate of introducing the vapor intothe distillation system (10), as first described above. The flowcontrolling device may be any known in the art. In one embodiment, theflow controlling device is a flow control valve which is automated andallows a rate of feed of the vapor to change with demand. The flowcontrolling device may be connected to a computer. In anotherembodiment, the flow controlling device is an orifice plate which is aflat piece of metal which defines an orifice. A size of the orificedetermines feed rate depending on a differential pressure across theorifice plate. In still another embodiment, the flow controlling deviceis a manual block valve. When the flow controlling device is utilized,the method typically further comprises the step of controlling a rate ofintroducing the vapor into the distillation system (10) as a function ofan amount of condensate distributed into the upper end (16) of thedistillation tower (14) from the reflux condenser (24) and as a functionof an amount of vapor formed from the reboiler (26).

The distillation system (10) may also include a neutralization tower(28) that is fluidly connected to the distillation tower (14). Thedistillate formed in the distillation tower (14) may be introduced intothe neutralization tower (28) or may be used for other purposes, such asthe formation of hydrogen chloride by using hydrogen in a “chlorineburner”, formation of phosgene for processes wherein an elevated brominelevel does not significantly impact quality of a final product, ordirect chlorination of other processes wherein bromine does notsignificantly impact a final product quality or overall yield, withoutuse of the neutralization tower (28).

The neutralization tower (28) may be of any size and shape as is knownin the art. Typically, the neutralization tower (28) has a shape as setforth in FIGS. 5 and 6, but is not limited to these shapes. Theneutralization tower (28) is typically a column having a first end (30)and a second end (32). The first end (30) and a second end (32)typically have varying widths, e.g. diameters (D₃, D₄), as shown inFIGS. 5 and 6. In other words, the neutralization tower (28) typicallyhas varying widths at different points. Typically, the varying widths ordiameters (D₃, D₄) are of from 6 inches to 5 feet. The neutralizationtower (28) typically has a height of from 1 meter to 100 meters. In oneembodiment, the first end (30) is cylindrical and has a diameter ofabout 10 inches. In another embodiment, the second end (32) is alsocylindrical and has a diameter of about 3 feet. In various embodiments,the neutralization tower (28) has a width and/or height ±60%, ±50%,±40%, ±30%, ±25%, ±20%, ±10%, ±5%, or any range therebetween, of theaforementioned values.

The first end (30) of the neutralization tower (28) is disposed abovethe second end (32) relative to gravity and the earth. Theneutralization tower (28) typically has a vertical axis (V₂) thatextends through the first and second end (30, 32), as shown in FIGS. 5and 6. Typically, the first and second ends (30, 32) extend along thevertical axis (V₂). In one embodiment, the neutralization tower (28) hasa third pairs of shoulders (38) that extend radially from the verticalaxis (V₂) and form an obtuse angle with the vertical axis (V₂). Theneutralization tower (28) may also have a horizontal axis (H₂) that mayor may not separate the first and second ends (30, 32)

The neutralization tower (28) may be the same as the distillation tower(14) or may be different. The neutralization tower (28) may have thesame dimensions described above relative to the distillation tower (14)or may have different dimensions.

Typically, the neutralization tower (28) is used to neutralize theliquid chlorine and the bromine component, e.g. the molecular bromineand the bromine monochloride. In one embodiment, the neutralizationtower (28) contains a reducing agent and a reducing catalyst. Typically,the reducing agent is selected from the group of metal hydroxides, metalsulfites, and combinations thereof. In one embodiment, the reducingagent is selected from the group of alkali metal hydroxides, alkalineearth metal hydroxides, and combinations thereof. In another embodiment,the reducing agent is selected from the group of alkali metal sulfites,alkaline earth metal sulfites, and combinations thereof. In yet anotherembodiment, the reducing agent is selected from the group of sodiumhydroxide (NaOH), sodium bisulfite (NaHSO₃), and combinations thereof.The reducing catalyst may be any known in the art and is typicallyfurther defined as a metal reducing catalyst. Typical metal reducingcatalysts include, but are not limited to, palladium, platinum, nickel,and combinations thereof. For descriptive purposes only, typicalreduction reactions (and other side reactions) of the liquid chlorine,the molecular bromine, and the bromine monochloride are set forth below:Br₂+Cl₂

2BrCl  (1)Br_(2(l))+2NaOH_((aq))→NaBr_((aq))+NaOBr_((aq))+H₂O_((l))  (2)3BrO⁻ _((aq))→2Br⁻ _((aq))+BrO₃ ⁻ _((aq))  (3)Cl_(2(aq))+2Br⁻ _((aq))→2Cl⁻ _((aq))+Br_(2(aq))  (4)Cl_(2(l))+2NaOH_((aq))→NaCl_((aq))+NaOCl_((aq))+H₂O_((l))  (5)3ClO⁻ _((aq))→2Cl⁻ _((aq))+ClO₃ ⁻ _((aq))  (6)Br_(2(aq))+2Cl⁻ _((aq))→2Br⁻ _(aq)+Cl_(2(aq))  (7)NaOBr_((aq))+NaHSO₃+NaOH→Na₂SO₄+NaBr+H₂O  (8)NaOCl_((aq))+NaOH→Na₂SO₄+NaCl+H₂O  (9)

In one embodiment, the liquid chlorine and bromine components areconverted to hypochlorites through treatment with caustic agents, suchas those described above. Hypochlorites can be decomposed to form alkalihalides and oxygen. Without intending to be bound by any particulartheory, it is believed that the primary chemical reaction may berepresented by the following equation:

In one embodiment, chlorine gas is reacted with sodium hydroxide to formsodium chloride and sodium hypochlorite that can then be furthertreated, such as with the Hydecat process, described below. Typically,the catalyst used to treat the sodium hypochlorite and sodium chlorideincludes a metal oxide or hydroxides of cobalt, copper, iron magnesium,molybdenum, and/or nickel. These catalysts may be supported orunsupported. The catalyst used to convert the sodium hypochlorite to thesodium chloride is not particularly limited in this invention. Withoutintending to be bound by any particular theory, it is believed thattime, temperature, pH and concentrations of catalyst and hypochloriteinfluence the reaction described above. Typically, below a pH of 7,hypochlorites readily decompose to release free chlorine.

In one embodiment, the instant invention utilizes a catalyst asdescribed in U.S. Pat. No. 4,764,286, which is expressly incorporatedherein by reference. In another embodiment, the instant inventionutilizes a catalyst as described in U.S. Pat. No. 4,963,341, which isalso expressly incorporated herein by reference. Moreover, the instantinvention may also utilize the Hydecat® process as described inWO9218235, which is also expressly incorporated herein by reference.

The distillation system (10) may also include additional components, asare known in the art. For example, the distillation system (10) mayinclude additional condensers, additional distillation tower (14),additional reboiler (26), and suitable valves, piping, and any othersuitable components described in Perry's Chemical Engineer's Handbook,McGraw-Hill Professional; 8th edition (Oct. 23, 2007), which isexpressly incorporated herein by reference relative to distillation anddistillation systems (10) and components.

Referring back to the method, the method also includes the step ofintroducing the vapor into the distillation system (10) to provide thepurified chlorine gas, the distillate including liquid chlorine and thebromine component, and a bottoms component including nitrogentrichloride. The method also includes the step of condensing the vaporin the reflux condenser (24) to form the condensate which flows from thereflux condenser (24) into the upper end (16) of the distillation tower(14) such that the condensate interacts with the vapor at thevapor-liquid contact device thereby forming the purified chlorine gasand the distillate. The method also includes the step of heating thecondensate in the reboiler (26). This step may decompose the nitrogentrichloride in the bottoms component. Moreover, the method includes thestep of removing the purified chlorine gas from the distillation system(10) and removing the distillate from the distillation system (10). Asdescribed above, the distillate may be removed from the distillationsystem (10) and introduced into the neutralization tower (28) or may beused for other purposes. In one embodiment, the vaporizer (12) adds heatinto the distillation system (10) such that the size of the distillationtower (14) and/or reboiler (26) can be reduced. In this embodiment, thereboiler (26) uses less energy than otherwise would be required becausethe vaporizer (12) heats the chlorine to the appropriate temperaturewhile the reboiler (26) maintains this temperature. In this embodiment,super-heated chlorine gas enters the distillation tower (14) from thevaporizer (12). Without intending to be bound by any particular theory,it is believed that this embodiment, and this invention, reduces energyconsumption and increases safety.

As described above, the distillate typically includes liquid chlorineand the bromine component and a bottoms component including nitrogentrichloride. In one embodiment, the distillate includes Br₂ and BrCl.The distillate is typically heated in the reboiler (26) which maydecompose the nitrogen trichloride. When the distillate is removed fromthe distillation tower (14), the bromine component is typically removedas well. The bromine component may be removed from the distillationsystem (10) at any temperature, as selected by one of skill in the art.In one embodiment, the distillation system (10) is in fluidcommunication with the neutralization tower (28), as described above,such that the bromine component is reduced and discarded. In anotherembodiment, the distillation system (10) is in fluid communication witha storage tank such that the bromine component may be used in variousdownstream applications.

The instant invention also provides a method of controlling a flow of avapor from the vaporizer (12) to the distillation system (10). In thisembodiment, the vapor is not particularly limited to a chlorine vapor,or the vapor described above, and may be any gaseous vapor. In thismethod, the reflux condenser (24), the reboiler (26), and the vaporinput are typically electronically coupled with a flow controllingdevice. The method includes the steps of introducing the vapor into thedistillation tower (14), condensing the vapor in the reflux condenser(24) to form the condensate, heating the condensate in the reboiler(26), and controlling a rate of introducing the vapor into thedistillation tower (14) as a function of an amount of condensate flowinginto the upper end (16) of the distillation tower (14) from the refluxcondenser (24) and as a function of an amount of vapor formed from thereboiler (26) to minimize a buildup of the condensate in thedistillation tower (14).

In one embodiment, the distillation system (10) is in fluidcommunication with a phosgene reactor such that the purified chlorinegas can be reacted with carbon monoxide to form phosgene gas(Cl₂+CO→COCl₂).

The instant invention also provides a method of forming an isocyanateincluding the reaction product of an amine and phosgene, which may beproduced as described immediately above. The isocyanate may includediisocyanates, polyisocyanates, biurets of isocyanates andpolyisocyanates, isocyanurates of isocyanates and polyisocyanates, andcombinations thereof. In one embodiment, the isocyanate is ann-functional isocyanate. In this embodiment, n is a number preferablyfrom 2 to 5, more preferably from 2 to 4, and most preferably from 2 to3. It is to be understood that n may be an integer or may haveintermediate values from 2 to 5. Alternatively, the isocyanate may beselected from the group of aromatic isocyanates, aliphatic isocyanates,and combinations thereof. In another embodiment, the isocyanate is analiphatic isocyanate such as hexamethylene diisocyanate or H12MDI. Theisocyanate component may also be further defined as a modifiedmultivalent aliphatic isocyanate, i.e., a product which is obtainedthrough chemical reactions of aliphatic diisocyanates and/or aliphaticpolyisocyanates. Examples include, but are not limited to, ureas,biurets, allophanates, carbodiimides, uretonimines, isocyanurates,urethane groups, dimers, trimers, and combinations thereof. In oneembodiment, the isocyanate is further defined as an aromatic isocyanate.Typically, aromatic isocyanates correspond to the formula R′(NCO)_(z)wherein R′ is aromatic and z is an integer that corresponds to thevalence of R′. Preferably, z is at least two. Suitable examples ofaromatic isocyanates include, but are not limited to,tetramethylxylylene diisocyanate (TMXDI), 1,4-diisocyanatobenzene,1,3-diisocyanato-o-xylene, 1,3-diisocyanato-p-xylene,1,3-diisocyanato-m-xylene, 2,4-diisocyanato-1-chlorobenzene,2,4-diisocyanato-1-nitro-benzene, 2,5-diisocyanato-1-nitrobenzene,m-phenylene diisocyanate, p-phenylene diisocyanate, 2,4-toluenediisocyanate, 2,6-toluene diisocyanate, mixtures of 2,4- and 2,6-toluenediisocyanate, 1,5-naphthalene diisocyanate, 1-methoxy-2,4-phenylenediisocyanate, 4,4′-diphenylmethane diisocyanate, 2,4′-diphenylmethanediisocyanate, 4,4′-biphenylene diisocyanate,3,3′-dimethyl-4,4′-diphenylmethane diisocyanate,3,3′-dimethyldiphenylmethane-4,4′-diisocyanate, triisocyanates such as4,4′,4″-triphenylmethane triisocyanate polymethylene polyphenylenepolyisocyanate and 2,4,6-toluene triisocyanate, tetraisocyanates such as4,4′-dimethyl-2,2′-5,5′-diphenylmethane tetraisocyanate, toluenediisocyanate, 2,2′-diphenylmethane diisocyanate, 2,4′-diphenylmethanediisocyanate, 4,4′-diphenylmethane diisocyanate, polymethylenepolyphenylene polyisocyanate, corresponding isomeric mixtures thereof,and combinations thereof. Alternatively, the aromatic isocyanate mayinclude a triisocyanate product of m-TMXDI and 1,1,1-trimethylolpropane,a reaction product of toluene diisocyanate and 1,1,1-trimethylolpropane,and combinations thereof. In one embodiment, the isocyanate is selectedfrom the group of methylene diphenyl diisocyanates, toluenediisocyanates, hexamethylene diisocyanates, H12MDIs, and combinationsthereof.

The amine that reacts with the phosgene to form the isocyanate may beany known in the art. In one embodiment, the amine reacts with thephosgene to form an intermediate isocyanate which then further reacts toform one or more of the isocyanates described above. Typically, aminesreacts with phosgene according to the following equation:RNH₂+COCl₂RN═C═O+2HClin the presence of a base such as pyridine.

The method of forming the isocyanate typically includes the steps ofintroducing the chlorine supply into the vaporizer (12), heating thechlorine supply in the vaporizer (12) to form the vapor, and introducingthe vapor into the distillation tower (14) that is fluidly connectedwith the vaporizer (12). The chlorine supply, the vaporizer (12), thevapor, and the distillation tower (14) are typically as described abovebut are not limited to the above descriptions. This method typicallyincludes the step of distilling the vapor in the distillation tower (14)to form the purified chlorine gas having less than 5 parts by weight ofthe bromine component per one million parts by weight of the purifiedchlorine gas. The method further typically includes the steps of heatingthe condensate and decomposing the nitrogen trichloride in the reboiler(26), removing the purified chlorine gas from the distillation tower(14), removing the distillate from the distillation tower (14), reactingthe purified chlorine gas with carbon monoxide to form the phosgene, andreacting the phosgene with the amine to form the isocyanate. The stepsof this method may be the same or different than those described above.

The invention has been described in an illustrative manner, and it is tobe understood that the terminology which has been used is intended to bein the nature of words of description rather than of limitation. Manymodifications and variations of the present invention are possible inlight of the above teachings, and the invention may be practicedotherwise than as specifically described.

It is to be understood that the appended claims are not limited toexpress and particular compounds, compositions, or methods described inthe detailed description, which may vary between particular embodimentswhich fall within the scope of the appended claims. With respect to anyMarkush groups relied upon herein for describing particular features oraspects of various embodiments, it is to be appreciated that different,special, and/or unexpected results may be obtained from each member ofthe respective Markush group independent from all other Markush members.Each member of a Markush group may be relied upon individually and or incombination and provides adequate support for specific embodimentswithin the scope of the appended claims.

It is also to be understood that any ranges and subranges relied upon indescribing various embodiments of the present invention independentlyand collectively fall within the scope of the appended claims, and areunderstood to describe and contemplate all ranges including whole and/orfractional values therein, even if such values are not expressly writtenherein. One of skill in the art readily recognizes that the enumeratedranges and subranges sufficiently describe and enable variousembodiments of the present invention, and such ranges and subranges maybe further delineated into relevant halves, thirds, quarters, fifths,and so on. As just one example, a range “of from 0.1 to 0.9” may befurther delineated into a lower third, i.e., from 0.1 to 0.3, a middlethird, i.e., from 0.4 to 0.6, and an upper third, i.e., from 0.7 to 0.9,which individually and collectively are within the scope of the appendedclaims, and may be relied upon individually and/or collectively andprovide adequate support for specific embodiments within the scope ofthe appended claims. In addition, with respect to the language whichdefines or modifies a range, such as “at least,” “greater than,” “lessthan,” “no more than,” and the like, it is to be understood that suchlanguage includes subranges and/or an upper or lower limit. As anotherexample, a range of “at least 10” inherently includes a subrange of fromat least 10 to 35, a subrange of from at least 10 to 25, a subrange offrom 25 to 35, and so on, and each subrange may be relied uponindividually and/or collectively and provides adequate support forspecific embodiments within the scope of the appended claims. Finally,an individual number within a disclosed range may be relied upon andprovides adequate support for specific embodiments within the scope ofthe appended claims. For example, a range “of from 1 to 9” includesvarious individual integers, such as 3, as well as individual numbersincluding a decimal point (or fraction), such as 4.1, which may berelied upon and provide adequate support for specific embodiments withinthe scope of the appended claims.

What is claimed is:
 1. A method for purifying a chlorine supplycomprising a chlorine component, a bromine component, and nitrogentrichloride in a distillation system to form purified chlorine gashaving less than 20 parts by weight of the bromine component per onemillion parts by weight of the purified chlorine gas and to form adistillate comprising liquid chlorine and the bromine component, whereinthe distillation system is fluidly connected to a vaporizer andcomprises a distillation tower that has an upper end and a lower end, avertical axis extending through the upper and lower ends, and avapor-liquid contact device to provide a vapor-liquid interface betweena vapor and a condensate, a reflux condenser that is in fluidcommunication with the upper end of the distillation tower, disposedabove the distillation tower, and that shares the vertical axis with thedistillation tower, and wherein the distillation system also comprises areboiler disposed below the distillation tower and in fluidcommunication with the lower end of the distillation tower, said methodcomprising the steps of: A. introducing the chlorine supply into thevaporizer; B. heating the chlorine supply to a temperature of from 50°C. to 120° C. in the vaporizer to form the vapor; C. introducing thevapor into the distillation system to provide purified chlorine gas, adistillate comprising the liquid chlorine and the bromine component, anda bottoms component comprising the nitrogen trichloride; D. condensingthe vapor in the reflux condenser to form the condensate which flowsfrom the reflux condenser into the upper end of the distillation towersuch that the condensate interacts with the vapor at vapor-liquidcontact device thereby forming the purified chlorine gas and thedistillate; E. heating the condensate in the reboiler; F. removing thepurified chlorine gas from the distillation system; and G. removing thedistillate from the distillation system.
 2. A method as set forth inclaim 1 wherein the chlorine supply further comprises water and carbontetrachloride.
 3. A method as set forth in claim 2 wherein the chlorinecomponent comprises liquid molecular chlorine and gaseous molecularchlorine.
 4. A method as set forth in claim 1 wherein the brominecomponent comprises molecular bromine and bromine monochloride.
 5. Amethod as set forth in claim 1 wherein the chlorine supply consistsessentially of the chlorine component, the bromine component, thenitrogen trichloride, water, and carbon tetrachloride, wherein thechlorine component comprises liquid molecular chlorine and gaseousmolecular chlorine, and wherein the bromine component comprisesmolecular bromine and bromine monochloride.
 6. A method as set forth inclaim 1 wherein the reflux condenser is further defined as a knockbackcondenser.
 7. A method as set forth in claim 1 wherein the upper end ofthe distillation tower defines a first orifice for removing the purifiedchlorine gas and the lower end of the distillation tower defines asecond orifice for removing the distillate, wherein the step of removingthe purified chlorine gas is further defined as removing the purifiedchlorine gas from the upper end of the distillation tower through thefirst orifice, and wherein the step of removing the distillate isfurther defined as removing the distillate from the lower end of thedistillation tower through the second orifice.
 8. A method as set forthin claim 7 further comprising the step of neutralizing the liquidchlorine and the bromine component.
 9. A method as set forth in claim 8wherein the step of neutralizing is further defined as reacting theliquid chlorine and the bromine component with a reducing agent in thepresence of a reducing catalyst to form salts.
 10. A method as set forthin claim 9 wherein the reducing agent is selected from the group ofmetal hydroxides, metal sulfites, and combinations thereof, and thereducing catalyst is a metal.
 11. A method as set forth in claim 10wherein the reducing agent comprises sodium hydroxide and sodiumbisulfite.
 12. A method as set forth in claim 1 wherein the step ofremoving the purified chlorine gas and the step of removing thedistillate occur simultaneously.
 13. A method as set forth in claim 1wherein the vapor-liquid contact device is further defined as aplurality of horizontal trays.
 14. A method as set forth in claim 1wherein the vapor-liquid contact device is further defined as structuredpacking.
 15. A method as set forth in claim 1 wherein the distillationtower is cylindrical and the upper end of the distillation tower has adiameter greater than a diameter of the lower end of the distillationtower to minimize an amount of the liquid chlorine and the nitrogentrichloride present in a bottom of the distillation tower.
 16. A methodas set forth in claim 15 wherein a ratio of the diameters of the upperand lower ends of the distillation tower is from 2:1 to 8:1.
 17. Amethod as set forth in claim 1 wherein the step of heating thecondensate in the reboiler is further defined as heating to atemperature of from 90° C. to 110° C.
 18. A method as set forth in claim1 wherein the reflux condenser, the reboiler, and a vapor input to thedistillation tower are electronically coupled to a flow controllingdevice for controlling a rate of introducing the vapor into thedistillation system and the method further comprises the step ofcontrolling a rate of introducing the vapor into the distillation systemthrough the vapor input as a function of an amount of condensate thatflows from the reflux condenser into the upper end of the distillationtower and as a function of an amount of vapor formed from the reboiler.19. A method of forming phosgene comprising the step of reacting carbonmonoxide and purified chlorine gas obtained from the method set forth inclaim
 1. 20. A method of forming an isocyanate comprising the reactionproduct of an amine and the phosgene formed from the method set forth inclaim
 19. 21. A method for purifying a chlorine supply consistingessentially of a chlorine component comprising molecular chlorine, abromine component comprising molecular bromine and bromine monochloride,nitrogen trichloride, water, and carbon tetrachloride, in a distillationsystem to form purified chlorine gas having less than 20 parts by weightof the bromine component per one million parts by weight of the purifiedchlorine gas and to form a distillate comprising liquid chlorine and thebromine component, wherein the distillation system is fluidly connectedto a vaporizer and comprises a distillation tower that has an upper end,a lower end, a and vertical axis extending through the upper and lowerends, wherein the upper end has a diameter that is greater than adiameter of the lower end, and wherein the distillation tower comprisesa plurality of horizontal trays to provide a vapor-liquid interfacebetween a vapor and a condensate, a knockback condenser disposed abovethe distillation tower, in fluid communication with the upper end of thedistillation tower, and sharing the vertical axis with the distillationtower, and a reboiler disposed below the distillation tower and in fluidcommunication with the lower end of the distillation tower, wherein thereflux condenser, the reboiler, and a vapor input to the distillationtower are electronically coupled to a flow controlling device forcontrolling a rate of introducing the vapor into the distillation systemand the method further comprises the step of controlling a rate ofintroducing the vapor into the distillation system through the vaporinput as a function of an amount of condensate that flows from theknockback condenser into the upper end of the distillation tower and asa function of an amount of vapor formed from the reboiler, said methodcomprising the steps of: A. introducing the chlorine supply into thevaporizer; B. heating the chlorine supply to a temperature of from 50°C. to 120° C. in the vaporizer to form the vapor; C. introducing thevapor into the distillation system to provide purified chlorine gas, adistillate comprising the liquid chlorine and the bromine component, anda bottoms component comprising the nitrogen trichloride such that a rateof introducing the vapor into the distillation system is controlled as afunction of an amount of condensate formed from the knockback condenserand as a function of an amount of vapor formed from the reboiler; D.condensing the vapor in the reflux condenser to form the condensatewhich flows from the reflux condenser into the upper end of thedistillation tower such that the condensate interacts with the vapor atvapor-liquid contact device thereby forming the purified chlorine gasand the distillate; E. heating the condensate in the reboiler to atemperature of from 90° C. to 110° C.; and F. simultaneously removingthe purified chlorine gas and the distillate from the distillationsystem.
 22. A method as set forth in claim 21 further comprising thestep of reacting the distillate and a reducing agent to form salts. 23.A method as set forth in claim 21 wherein the step of reacting thedistillate and the reducing agent is further defined as reacting thedistillate and the reducing agent in the present of a reducing catalystand wherein the reducing agent is selected from the group of metalhydroxides, metal sulfites, and combinations thereof, and the reducingcatalyst is a metal.
 24. A method of forming phosgene comprising thestep of reacting carbon monoxide and the purified chlorine gas obtainedfrom the method set forth in claim 22.